1. Field of the Invention
The invention relates to semiconductor wafer processing equipment and, more particularly, the invention relates to a return path for RF current within such equipment.
2. Description of the Background Art
Plasma-enhanced reactions and processes have become increasingly important to the semiconductor industry, providing for precisely controlled thin-film depositions.
FIG. 1 depicts a cross-sectional, simplified view of a conventional physical vapor deposition (PVD) wafer processing chamber 100 of the prior art. The chamber 100 comprises a set of walls that define a volume having a conventional pedestal assembly 102 positioned in the volume. The pedestal assembly 102 comprises a pedestal 106 and a susceptor 107. The susceptor 107 has a surface 114 that supports a wafer 104. A chamber lid 110 at the top of the chamber 100 contains deposition target material (e.g., titanium) and is negatively biased by a DC source 119 to form a cathode. Alternately, a separate target is suspended from the chamber lid 110. The chamber lid 110 is electrically insulated from the remainder of the chamber 100. Specifically, an insulator ring 112, electrically isolates the chamber lid 110 from a grounded annular shield member 134 which forms an anode. The pedestal assembly 102 has a range of vertical motion within the chamber 100 to facilitate wafer transfer. The pedestal assembly is depicted in a raised position (wafer processing position) in FIG. 1. The chamber includes a ring assembly 118 that prevents deposition from occurring in unwanted locations such as upon the sides of the susceptor, beneath the pedestal and the like. Specifically, a waste ring 120 and cover ring 122 prevent sputtered material from being deposited on surfaces other than the substrate.
An electric field is induced in a reaction zone 108 between the cathode chamber lid 110 and anode shield member 134 when the DC source 119 is switched on. A process gas such as argon is provided to the reaction zone 108 via a working process gas supply (not shown). The electric field created by the high power DC source 119 ionizes the process gas and creates a uniform, high-density, low electron temperature plasma 116. The grounded shield member 134 surrounds a reaction zone 108 and confines the plasma 116 to enhance deposition.
To further enhance deposition in an ion metallization system, a specific type of PVD system, the substrate 104 and susceptor 107 are biased negatively with respect to the plasma 116. This is accomplished by providing RF power to an electrode 130 within the pedestal assembly 102. Ordinarily, a 400 KHz AC source 136 is used to bias the substrate 104, but other frequency sources such as a 13.56 MHz source may also be used. A negative DC potential (i.e., a bias voltage) accumulates on the substrate 104 as a result of the higher velocity of electrons as compared to the positive ions in the plasma 116. In some PVD processes, as neutral target material is sputtered from the target and enters the plasma 116, the target material becomes positively ionized. With the negative DC offset at the substrate, the positively ionized target material is attracted to and deposits on the substrate in a highly perpendicular manner. That is, the horizontal component of acceleration and/or velocity of the positive ion is reduced while the vertical component is enhanced. As such, the deposition characteristic known as "step coverage" is improved.
Ideally, the bias voltage on the substrate 104 (i.e., a semiconductor wafer) remains stable as the target material is being deposited onto the substrate 104. A stable voltage level at the substrate 104 causes the ionized deposition material to be drawn uniformly to the substrate 104. A uniform deposition film layer is a highly desirable characteristic in the semiconductor wafer manufacturing industry. Voltage stability is optimized when there is no appreciable voltage drop due to current flowing in the return path from the shield member 134 to ground.
In the prior art, the ground path for RF current is rather circuitous. For example, the substrate 104 is in electrical contact with the plasma 116 which is in electrical contact with the shield member 134. The shield member 134 is connected to the chamber wall 103. The chamber wall 103, in turn, is connected to the pedestal 106 through a flexible bellows 138. The pedestal is connected to ground through a tube 140 that runs inside the bellows 138. Typically, the bellows 138 are made of thin stainless steel discs welded together. The discs are very thin and stainless steel has a relatively low conductivity. This is not a problem for DC currents since the voltage drop over the return path is small. However, for RF applications, currents of approximately 20 to 30 amps are common. The stainless steel bellows 138 have a high RF impedance. As such, the bellows are unsuitable as a return path for RF currents since a large voltage drop develops across the bellows during processing. Such a large voltage drop, induced by the large impedance of the return current path, causes high voltages on the surface of the pedestal 106. Plasma can strike between two points at substantially different voltages and lead to stray plasma in the chamber. For example, such an unwanted plasma can strike between the pedestal 106 at a high potential and some other nearby grounded feature such as the shield member 134, the chamber walls 103 or bake out lamps (not shown). The stray plasma spreads out to fill all of the space outside the reaction zone 108 (i.e., the region between the pedestal 106, the bellows 138, the shield member 134 and the chamber walls 103). The stray plasma may sputter material from the bellows 138 and pedestal 106 introducing contaminants into the chamber environment as well as reducing the life of the pedestal assembly.
In a 300 mm wafer processing system the path to ground is especially long and the voltages induced are quite high (e.g., approximately 700 volts peak to peak). As such, the voltage on the wafer 104 becomes unstable and non-reproducible. The long return path also creates a variable impedance that changes after each repositioning. A ground path could be made between the shield member 134 and the pedestal 106 via the waste ring 120 and the cover ring 122. However, this path would be broken each time the pedestal assembly 102 is lowered and raised during wafer transfer and, therefore, would be unreliable.
Therefore, a need exists in the art for reliable low impedance return path for RF current to ensure wafer voltage stability and uniformity of deposition.